A collector mirror for an EUV microlithography system is known from US 2009/0267003 A1.
A collector mirror of this type is used in EUV microlithography, which involves a method for producing and structuring semiconductor components, integrated circuits and components appertaining to micro- and nanosystems engineering. In microlithography, structures that are predefined on a reticle are imaged onto a substrate, for example a silicon substrate, with the aid of exposure processes. In this case, the substrate is coated with a light-sensitive material, which is generally known as “photoresist”. During exposure, the exposure light interacts with the photoresist layer, with the result that the chemical properties of the exposed regions of the photoresist layer change. In a subsequent development step, the photoresist dissolves in the exposed or non-exposed regions, depending on whether a “positive” or a “negative” photoresist is used. Finally, the regions of the substrate surface which are not covered by the remaining photoresist are removed in an etching method, for example wet-chemical etching, plasma etching or plasma-assisted reactive ion etching. This gives rise to the predefined structures of the reticle on the processed substrate surface on a projection scale that is characteristic of the microlithography system used, in particular a projection exposure apparatus.
The performance of the microlithographically produced semiconductor components or integrated circuits is all the higher, the higher the integration density of the structures in the components. In other words, one endeavor consists in imaging increasingly finer structures onto the substrate. The lower limit of the structure size achievable in optical lithography is determined, inter alia, by the wavelength of the exposure light used. Therefore, it is advantageous to use exposure light having the shortest possible wavelength. With regard to this aspect, projection exposure apparatuses are known which use extreme ultraviolet (EUV) light as exposure light, the wavelength of which is 13.5 nm.
The EUV light is generated by an EUV light source in which a plasma is generated by strong electrical discharges (referred to as: Gas Discharge Produced Plasma, GDPP) or by focusing of laser radiation (referred to as: Laser-Produced Plasma LPP). In the LPP method, a tin droplet is bombarded with pump light, wherein infrared (IR) light is usually used as pump light. The generated plasma contains a multiplicity of charge particles, for example electrons, which fall from energetically high states to energetically lower states and emit the desired EUV light in the process. Furthermore, on account of the high temperature prevailing in the plasma, for example above 200,000 K, EUV light can be emitted in the form of black body radiation. The EUV light generated propagates in all spatial directions. In order that the EUV light becomes usable as used rays for the exposure process, the largest possible portion thereof is directed in the direction of the illumination and projection optical unit by a collecting optical unit, which is known by the term “collector mirror”.
Collector mirrors known from the prior art include an elliptical mirror surface for the purpose of better light focusing. However, they have the disadvantage that, as the density of the tin plasma increases, the plasma frequency can increase greatly. In this case, besides the used rays, remaining rays, i.e. electromagnetic rays from a remaining spectral range different than the EUV spectral range, which are not usable for the exposure process, may also be reflected at the mirror surface of the collector mirror and be focused onto the second focal point. Finally, the remaining rays may pass into the illumination and imaging optical unit and even as far as the substrate. The remaining rays typically include highly intensive IR light and deep ultraviolet (DUV) light. The remaining rays cause a considerable input of heat to the optical units disposed downstream of the collector mirror and thus an unacceptable impairment of the optical properties of these optical units.
In order to address this issue, further elliptical collector mirrors are known which are embodied as Spectral Purity Filters (SPF) and include a binary grating. The binary grating serves to reflect the EUV light emitted at the first focal point and to focus it onto a second focal point, wherein the focused EUV light passes through a stop situated at the second focal point, while the IR pump light is blocked there. As a result, the IR pump light is suppressed.
However, collector mirrors of this type can have the disadvantage that the binary grating can effectively suppress only remaining rays of a single wavelength, wherein this is possible moreover only if the grating parameters of the binary grating, for example the grating period, were controlled very accurately during manufacture. Additional manufacturing outlay arises here. In addition, the remaining rays in a broadband remaining spectral range have to be suppressed in the imaging beam path, which additionally increases the manufacturing outlay.
Furthermore, the binary grating can have surfaces roughnesses with dimensions in the range of medium-wave wavelengths. Such surface roughnesses are attributable to vibrations and accuracy limitations of the devices used for producing the binary grating. They have the effect that the EUV light generated is not focused exactly onto the second focal point of the collector mirror, but rather onto an extended region around the second focal point. The second focal point for the EUV light is also referred to as “intermediate focal point (IF)”. A widening of the IF occurs undesirably. In order to allow the used rays to pass through the stop as completely as possible, the aperture of the stop is correspondingly enlarged. However, this leads to increased transmission of the remaining rays.
A further possible disadvantage of such SPF collector mirrors is that the collector mirror has to be formed with a very large diameter in order to take account of the desire for a large working distance and a small IF-side numerical aperture (NA) simultaneously. The larger the diameter of the collector mirror, the greater the extent to which the imaging scale of the collector mirror varies along the elliptical mirror surface. The imaging scale is defined as the ratio between the distance from the first focal point to the ray incidence point on the mirror surface, on the one hand, and the distance from the ray incidence point to the second focal point, on the other hand. This leads undesirably to an enlargement of the envelope of the etendue, in particular at the IF and/or in the far field. The abovementioned surface roughnesses, too, additionally foster the enlargement of the envelope of the etendue.
An enlargement of the envelope of the etendue can cause a loss of the used rays generated, which has a serious effect on the exposure quality and/or the imaging quality in extreme illumination modes of an EUV microlithography system.
US 2009/0267003 A1 discloses further elliptical collector mirrors including a blazed grating including a plurality of mirror facets. It is true that the used rays can thereby be focused with an increased transmission efficiency. Nevertheless, issues associated with the possible disadvantages mentioned above, for example the loss of used rays, remain very largely unsolved.
DE 10 2013 002 064 A1 discloses a collector mirror for collecting EUV radiation, which collector mirror includes an optical grating including a plurality of grating elements. The grating elements may be embodied such that they constitute sections of ellipsoids.